WO2006032612A1

WO2006032612A1 - Method for testing a magnetic inductive flow meter

Info

Publication number: WO2006032612A1
Application number: PCT/EP2005/054427
Authority: WO
Inventors: Thomas Budmiger
Original assignee: Endress+Hauser Flowtec Ag
Priority date: 2004-09-22
Filing date: 2005-09-07
Publication date: 2006-03-30
Also published as: CN100451563C; RU2007115055A; DE102004046238A1; EP1792144A1; RU2356014C2; CN101057125A; EP1792144B1

Abstract

The inventive flow meter comprises a measuring tube for conducting a fluid to be measured and a magnetic field system with at least one field coil that is at least temporarily traversed by an excitation current (IM) for generating a magnetic field that penetrates the fluid at least partially and runs perpendicular to a flow direction. According to the invention, the voltage (UH) that is currently supplying the excitation current (IM) is modified at a second point in time t2 from the second voltage level Udrv to a third voltage level Ushort that is in particular constant, or regulated to remain constant, in order to reduce the current intensity of the excitation current (IM) that is flowing in the field coil(s) from a maximum current value Im to a final, in particular constant, current value Izu that is predetermined for the excitation current (IM). The third current level Ushort is lower than the second current level Urev. To determine a third point in time t3, which corresponds to the attainment of the final current value Ic, the excitation current IM is at least intermittently recorded. Based on said recording, a first time constant Tshort is determined for the magnetic field system, said constant corresponding to a time period t3 - t2 lying between the second point in time t2 and the third point in time t3 and/or a second time constant Trev + Tshort is determined for the magnetic field system, said constant corresponding to a time period t3 - t1 lying between the first point in time t1 and the third point in time t3. To determine a diagnostic value that ultimately represents the current operating condition of the flow meter, the first determined time constant Tshort is compared to a predefined first reference value T1ref and/or the second determined time constant Trev + Tshort is compared to a predefined second reference value T2ref.

Description

description

Method for checking a magneto-inductive Durch¬ flow meter

The invention relates to a method for checking a magneto-inductive

Flow meter with a measuring tube for guiding a fluid to be measured and with a magnetic field system having at least one at least temporarily energized by a Er¬ current field coil for generating a fluid at least partially perpendicular to a flow direction passing magnetic field.

For the measurement of electrically conductive fluids flow meter are often used with a magnetic-inductive flow sensor. In the following, if necessary, for the sake of simplicity, only flow meters or flow meters will be discussed. Magnetic-inductive flow meters can be used to measure the volume flow rate of an electrically conductive liquid and to map it into a corresponding measured value which flows in a pipeline; By definition, therefore, the volume of liquid flowing per unit time through a tube cross-section is measured. Structure and mode of operation of magnetic-inductive Durch¬ flow meter are known in the art and in itself and for example in DE-A 43 26 991, EP-Al 275 940, EP-A 12 73 892, EP-A 1 273 891 , EP-A 814 324, EP-A 770 855, EP-A 521 169, US-A 60 31 740, US-A 54 87 310, US-A 52 10 496, the

US-A 44 10 926, US-A 2002/0117009 or WO-A 01/90702 described in detail and in detail.

Flow sensors of the type described usually each have a non-ferromagnetic measuring tube which is fluid-tight in the pipeline, for example by means of flanges or screw connections. An electrical voltage generated in the flowing fluid due to charge separations by means of a magnetic field oriented transversely to a flow direction of the fluid to be measured is tapped by means of at least two measuring electrodes as a measuring voltage and further processed in the measuring device electronics to a corresponding measured value, for example a volumetric flow measured value. The fluid contacting part of the measuring tube is generally electrically non-conductive, so that the measuring voltage, which is induced by the Faraday's law of induction of the measuring tube at least partially durch¬ putting magnetic field in the fluid is not short-circuited. Metal measuring tubes are therefore usually internally provided with an electrically non-conductive layer, for example of hard rubber, polyfluoroethylene, etc., and also generally non-ferromagnetic; completely made of a plastic or a ceramic, esp. made of alumina ceramic, existing measuring tubes, the electrically non-conductive layer is not required in contrast.

In the case of magnetic-inductive flow sensors, the magnetic field required for the measurement is generated by a corresponding magnetic field system which has a coil arrangement having an inductance L and usually at least two field coils, corresponding coil cores and / or pole shoes for the field coils and, if appropriate, the coil cores containing outside the measuring tube connecting magnetic-conductive Füh¬ ments sheets. However, magnetic field systems with a single field coil are also known. The coil cores and / or pole pieces of the magnet system usually consist of a soft-magnetic material. However, magnet systems with ferromagnetic coil cores have also already been described. The magnetic field system is usually arranged directly measuring tube.

[006] To generate the magnetic field, a signal from a corresponding measuring device

Electronics delivered coil current flow in the coil assembly. So that the magnetic field generated by the magnetic field system is as homogeneous as possible, the field coils are in the most common and simplest case identical to each other and electrically connected in series in series, so that they can be flowed through during operation of the same exciter current. However, it has also already been described to allow the coils to flow alternately in the same direction or in opposite directions from an exciter current, in order thereby to be able to determine, for example, the viscosity of liquids and / or a degree of turbulence of the flow, cf. for this purpose also EP-A1 275 940, EP-A 770 855 or DE-A 43 26 991. The aforementioned exciting current is generated by an operating electronics; it is set to a constant current value of e.g. 85 mA regulated, and its current direction is reversed periodically. The current direction reversal is achieved in that the coils are in a so-called T-circuit or a so-called H-circuit; for current regulation and direction reversal cf. the US-A 44 10 926 or

US-A 60 31 740.

The coil current is in modern Durchflußaufnehmern usually a clocked bi-polar square-wave alternating current, which is in a first half period of a period Pe¬ positive with a constant first current end value and then by switching in a second half period of the period negative with a constant, to the first current end value substantially the same amount second current end value. The coil arrangement may for example be a single coil, if the magnetic-inductive flow sensor serves as a flow probe, cf. US-A 35 29 591, or it may for example consist of two part-coils which are arranged diametrically opposite to each other on the measuring tube. In the US-B 67 63 729, US-A 60 31 740 or US-A 44 10 926 is a circuit arrangement for Generation of such a coil current described. This circuit arrangement comprises a power supply which drives the coil current and a bridge circuit designed as an H circuit for the modulation of the coil current, the coil arrangement being located in a shunt branch of the bridge circuit. Furthermore, in US-A 60 31 740 as well as in US-A 44 10 926 a circuit arrangement for generating a coil current is shown, the circuit instead of the designed as H-circuit Brücken¬ designed as a T-circuit bridge circuit for the coil assembly , Furthermore, in the

US-A 42 04 240 a circuit arrangement with an internal power supply for generating the coil current of a magnetic-inductive flow sensor described which outputs a voltage from the voltage initial value in each half-period mentioned during a rise time of the coil current - as a first sub-period - higher than a voltage end value during a second sub-period - as the remainder of the half-period.

[010] The mentioned induced voltage arises between at least two galvanic, ie wetted by the liquid, or between at least two capacitors, thus e.g. arranged within the tube wall of the measuring tube, Meß¬ electrodes, each of the electrodes picks off a potential for itself. In the most common case, the measuring electrodes are arranged diametrically opposite one another so that their common diameter is perpendicular to the direction of the magnetic field and thus perpendicular to the diameter on which the coil arrangements lie. The induced voltage is amplified and processed by means of an evaluation circuit to a measurement signal, which is registered, displayed or in turn further processed. Corresponding measuring electronics are likewise known to the person skilled in the art, for example from EP-A 814 324, which

[011] EP-A 521 169 or WO-A 01/90702.

In addition to the actual measuring function, modern magneto-inductive flow meters often also have superordinate diagnostic functions by means of which the flowmeter can be subjected to a self-test during operation. Such diagnostic functions are described, for example, in the already mentioned US Pat. No. 6,763,729 or EP-A 12 17 337.

In such a self-test, on the one hand, a check can be made as to whether all components of the measuring instrument are fully functional, and on the other hand, whether the measuring operation is proceeding according to the regulations. If the result of such a self-test is positive, ie if there are no malfunctions of the measuring instrument or of the measuring operation, then it is ensured that the measured value output by the measuring instrument-of course within predefined fault tolerances-corresponds to the currently actually present value. In particular, under a such self-tests individual components and components of the measuring device, for example by means of impedance measurements or measurements of the ohmic resistance or the conductivity are checked for their functionality. Furthermore, it is also z. B. to check the Ein¬ stelldaten when entering plausibility. For example, entries that are outside the permitted range are rejected and not accepted. Independently of the check of the setting data during the input, the data calculated from the setting data, which directly control the measuring operation of the measuring device, are checked again for their permitted limits. This check is carried out before starting the measuring operation, so that when an error occurs with respect to the setting data, a corresponding error message can be output and the measurement is not carried out accordingly.

In the measuring device described in EP-A 12 17 337 is proposed the

To keep exciting current variable in its current, to measure the exciting current ent¬ speaking measuring voltage and due to a threshold value über¬ increasing deviation measured at different current Meßspan- heights of the expected by the function between the excitation current and the measuring voltage for the voltage level a malfunction of the measuring operation of the meter is detected. Thus, the function, which is known as pre-exposed and fundamental for the measuring operation, between the excitation current and the measuring voltage is used to test the measuring operation. In the simplest case, this function between the exciting current and the measuring voltage may be a linear function, so that a malfunction of the measuring operation of the measuring device by simply varying the measured voltage levels obtained at mutually different current levels by more than a threshold value of a linear Course is detectable.

On the other hand, in the flow meter described in US Pat. No. 6,763,729, the check is carried out by determining at least one parameter of the rise of the current after the change of the current direction and comparing this with a reference value. In this case, the parameter used is a period of time which elapses between two predetermined current values. Since the increase of the current satisfies a predetermined physical law, usually an e-function, it is sufficient to determine the rise time between two values in order to obtain a reliable statement about the current increase per se. Alternatively or additionally, the parameter used may be a period of time which elapses between the switching of the current direction and the reaching of a predetermined current value. The timing of switching is very accurate to determine. For example, the switching signal can also be used as a trigger signal for a time counter. The predetermined current value may be close to, for example are the maximum current value, ie in the vicinity of the current, which adjusts after the settling of the magnetic field to a suitable value for the measurement constant value. Furthermore, in US-B 67 63 729 after switching to use an increased voltage. As already disclosed in US-A 60 31 740, such an overvoltage accelerates the build-up of the magnetic field, since it increases the current increase, thus allowing the actual measurement to be made faster again.

[016] The threshold or reference value, when exceeded, a malfunction of the

Measuring operation of the meter is detected can be determined in the flow meter itself at an earlier time. For example, during commissioning, the desired parameter can be determined and stored as a reference value in a provided in the meter electronics microcomputer as a digital value, so that it is available for future verification operations. This gives each Durch¬ flow meter an individual reference value. The reference value for the deviation may be determined in different ways, e.g. B. as a constant absolute value. Furthermore, it is possible to set the threshold value as a percentage of the measuring range end value for the measuring voltage. In this way it can be achieved by setting the threshold once - namely as a percentage of the Meßbereichsendwert for the output - to achieve a setting of the meter that can be used for all adjustable ranges of the meter.

Incidentally, in determining a malfunction of the measuring operation of the measuring apparatus, various operations are possible. For example, the measuring operation of the meter can be terminated, since no more reliable measurement results are achieved. Alternatively or additionally, however, visual and / or audible warning messages can also be output when determining a malfunction of the measuring operation.

[018] In spite of these prior art methods of self-testing measuring instruments, there is a need to provide further self-testing methods that can more accurately determine whether there is a malfunction of the measuring operation or the meter itself. In particular, namely the case may occur that individual components of the meter differ only slightly from their nominal values, so that each component is classified as fully functional in a self-test, but in the interaction of the components such a measurement operation is generated, which is faulty and at the specified accuracy of the measured values can not be guaranteed.

Moreover, it has also been found that, owing to the fact that, in the magnetic system, due to the switching and the increase of the coil current induced currents, which, as discussed in US-A 60 31 740, prevent the increase of the magnetic field follows exactly the increase of the coil current, as would be the case without coil cores and / or pole shoes. Rather, the increase of the magnetic field is delayed and flattened compared to the coil current.

Ideally, the current flow in the coil arrangement corresponds to the course of the

Magnetic field. Due to eddy currents that arise during the switching of the magnetic field in the pole shoes and cores of the coil arrangement, deviations from the ideal case occur in reality. The coil current measured outside the coil arrangement therefore corresponds to the sum of the current flowing in the coil and the current generated by the eddy currents. If the current measured outside the coil arrangement is used as the controlled variable, then the current is not constant but the magnetic field is constant. This applies as long as the eddy currents have not decayed.

This adverse effect of the eddy currents also occurs and despite the mentioned

Voltage overshoot. The effect of the eddy currents can be illustrated by assuming that the (coil) inductance is connected in parallel to an eddy current source whose current adds to the current in the inductance to the total exciter current. Thus, although the voltage drop across a resistor in series with the inductance of the field coil is a measure of the excitation current, it does not account for the true field current in the inductance actually corresponding to the magnetic field. However, this is, as also discussed in EP-A 14 60 394, required for an exact control of the magnetic field and as far as for the review of the magnetic field system of advantage.

[022] It is therefore an object of the invention to provide a method with which a

Flow meter of the type described during the current measurement operation can be checked and insofar malfunction of the meter can be detected very accurately and easily and possibly also localized.

To solve the problem, the invention consists in a method for testing a magnetic-inductive flow meter with a measuring tube for guiding a fluid to be measured and with a magnetic field system, the at least one at least temporarily traversed by an excitation current field coil for generating a the Fluid having at least partially perpendicular to a flow direction durch¬ putting magnetic field, which method comprises the following steps:

[024] changing a voltage that drives the excitation current at least temporarily at a first time, from a momentary, in particular zero, first voltage level to an, in particular constant or constantly regulated, second voltage level for increasing a current intensity of the voltage driven and flowing in the at least one field coil excitation current to a maximum Current value, wherein the second voltage level is chosen to be greater than the first voltage level,

[025] Change the voltage currently driving the exciting current to a second one

Time from the second voltage level to one, esp. Constant or konstant¬ regulated, third voltage level for lowering the current intensity of the currently flowing in the at least one field coil excitation current from the maximum current value to a predetermined for the excitation current, esp. Constant, current end value wherein the third voltage level is selected smaller than the second voltage level

[026] - Detecting the excitation current at least temporarily to determine a third

Time that corresponds to reaching the final current value,

Determining a first time constant for the magnetic field system that corresponds to a time period lying between the second time point and the third time point, and / or determining a second time constant for the magnetic field system, which one with the one between the first Time and the third time corresponds time, and

[028] - comparing the determined first time constant with a predetermined first

Reference value and / or comparison of the determined second time constant with a predetermined second reference value for determining a diagnosis value representing a momentary operating state of the flow meter.

A first embodiment of the method according to the invention provides that the third voltage level is chosen smaller than the first voltage level.

[030] A second embodiment of the method according to the invention includes that the voltage driving the excitation current at least temporarily is changed such that its second voltage level during a period of time lying between the first time and the second time at least temporarily, esp. Also immediately before the second time , is substantially constant.

A third embodiment of the method according to the invention provides that at least one field coil of the coil arrangement is at least temporarily short-circuited to set the voltage to the third voltage level.

[032] A basic idea of the invention is the third point in time, which reacts to a very great extent as a function of the instantaneous operating state of the flow meter, but in particular of the instantaneous state of the magnetic field system and is therefore very sensitive to changes in the magnetic field system - It is meant here in particular the time period measured by a suitable reference time point - of the method described above for exciting current or magnetic field setting for monitoring the flow meter, esp. Of the magnetic field system to use. The invention is based inter alia on the finding that the error in the magnetic field system has a significant influence on the two mentioned periods of time have the regulation for the excitation current.

The method of the invention and further advantages will now be described with reference to FIGS

Figures of the drawing illustrated timing diagrams and schematized diagrams of a magnetic-inductive flowmeter explained in more detail.

Figs. 1, 2 show schematically and in the form of a block diagram one for the

Process of the invention suitable - here magnetic-inductive Durch¬ flow meter trained - process measuring device,

3 schematically shows a time diagram of an excitation current flowing during operation of the process measuring device according to FIGS. 1 and 2,

FIG. 4 shows a schematic circuit diagram of a first embodiment with an H circuit, FIG.

FIG. 5 shows a schematic circuit diagram of a second embodiment with an H circuit, FIG.

FIG. 6 shows a schematic circuit diagram of a first exemplary embodiment with a T-circuit, FIG.

FIG. 7 shows a schematic circuit diagram of a second embodiment with a T-circuit, FIG.

FIG. 8 shows waveforms of the coil current and the magnetic induction or the voltage of the voltage source, FIG.

[041] FIG. 9 shows a soot diagram of a micro-processor used in the invention.

10 shows a graphical representation of the time course of the coil current and the time course of the magnetic field in the solution known from the prior art,

11 shows a graph of the time profile of the voltage applied to the coil arrangement in the solution known from the prior art, FIG.

[044] FIG. 12 is a graphic representation of the time profile of the voltage applied to the coil arrangement in the device according to the invention and FIG

[045] FIG. 13 is a graph showing the time course of the coil current and the time course of the magnetic field in the device according to the invention.

1 shows schematically and partly in the form of a block diagram an electromagnetic flowmeter suitable for the process of the invention, by means of which measured values for at least one physical variable of a pipeline are shown here flowing medium, esp. Of a fluid can be generated. For example, the flowmeter may be used to measure a volumetric flow rate and / or a flow rate of an electrically conductive fluid. [047] The flowmeter shown here comprises a flow sensor 1 for

Generating Meß¬ potentials corresponding to the physical quantity to be measured, a measuring device having a microcomputer with a, especially also at least partially realized by the microcomputer, measuring and operating circuit 2 for detecting the measuring potentials and generating at least one with the physical Size corresponding measuring signal and an, especially by means of the microcomputer realized, evaluation circuit 3, which serves to control the measuring and operating circuit 2 and thus the flow sensor 1 and to generate using the at least one measurement signal, the physical size reprä¬ sent measurements , The measuring and operating circuit 2 and possibly also some components of the Durchflußaufnehmers 1, for example, as shown schematically in Fig. 2, be housed in an electronics housing 10 of the flow meter.

For Durchflußaufnehmer 1 includes an insertable in the course of the aforementioned pipe measuring tube 11, which has a tube wall and is allowed to flow through the operating in the direction of a Meßrohrlängsachse the fluid to be measured.

In order to avoid a short-circuiting of stresses induced in the fluid, an inner part of the measuring tube 11 contacting the fluid is made electrically non-conductive. Metal measuring tubes are usually for this purpose inside with an electrically non-conductive layer, e.g. made of hard rubber, polyfluoroethylene, etc., provided and also in general non-ferromagnetic; when completely made of a plastic or ceramic, esp. Alumina ceramic, existing measuring tubes, the electrically non-conductive layer is not required in contrast.

[050] One of a driver circuit provided in the measuring and operating circuit 2

Electronics 21 controlled coil arrangement of a magnetic field system of Durch¬ flow meter has in the illustrated embodiment, a measuring tube 11 arranged on the first field coil 12 and arranged on the measuring tube 11 second field coil 13. The field coils 12, 13 are located on a first diameter of the measuring tube 11. The magnetic field system is used in conjunction with the meter electronics to generate a magnetic field B passing through the tube wall and the fluid flowing through it. This occurs when, in the field coils 12, 13 connected in series here, an excitation current I driven by the driver electronics 21 is allowed to flow. The, in particular bi-polar, exciting current I can in this case e.g. be formed substantially rectangular or at least rectangular-like.

[051] It is shown in FIG. 3 that the field coils 12, 13 do not contain a core, that is to say so-called air coils. However, the field coils 12, 13 can also, as is usual with such coil arrangements, be wound around a core, which is generally soft-magnetic, wherein the cores can cooperate with pole shoes, cf. eg US-A 55 40 103.

The coil arrangement shown in the embodiment shown here is embodied here, esp. The two field coils 12, 13 are shaped and dimensioned so that the magnetic field H generated thereby within the measuring tube 11 at least with respect to a second diameter perpendicular to the first diameter symmetrical, esp is rotationally symmetrical, is formed.

According to one embodiment of the invention, by means of the driver electronics 21 a, in particular. Constant amplitude regulated, DC generated, which then switched by means of a corresponding, for example configured in H or T circuit, switching mechanism periodically and so to a AC is modulated with controlled amplitude. As a result, therefore, the exciting current I is flowed through the Spulen¬ arrangement that the coils 12, 13, as shown schematically in Fig. 2a during a first switching phase PHl 1 each in a first current direction and during a second switching phase following the first switching phase PH12 are respectively flowed through in a Ge direction opposite to the first current direction, cf. for current regulation and reversal, e.g. also US-A 44 10 926 or

[054] US-A 60 31 740.

The second switching phase PH12 is followed by a third switching phase PH21, during which the excitation current I flows again in the first current direction. The third switching phase is followed by a fourth switching phase PH22, during which the exciter current I flows again in the direction opposite to Ge. This is followed by a corresponding switching phase PH31 and so on. With respect to the reversal of the direction of the excitation current I, two of the successive switching phases each form a switching period P1, P2, P3 and so on. Along with the polarity reversal of the exciting current flowing through the coil arrangement I, apart from a possible switching phase shift essentially synchronously with it, the magnetic field H is also repeatedly reversed, cf. see Fig. 2a.

[056] For generating at least one electric current corresponding to the measured quantity

Measuring signal is furthermore provided on the measuring tube or at least in its vicinity arranged sensor arrangement in the transducer. According to one embodiment of the invention, the sensor arrangement has electrodes mounted directly on the measuring tube. A first electrode 14 arranged on an inner side of the tube wall of the measuring tube 11 serves to pick off a first potential e induced by the magnetic field H. A second electrode 15 arranged in the same way also serves to pick off a second potential e induced by the magnetic field. The Meß¬ electrodes 14, 15 lie on the first diameter and the Meßrohlängsachse perpendicular second diameter of the measuring tube 11, but they can for example also lie on a parallel to the second diameter chord of the measuring tube 11, see. For this also US Pat. No. 5,646,353.

In FIG. 1b, the measuring electrodes 14, 15 are shown as galvanic measuring electrodes, ie as those which touch the fluid. However, two capacitive, e.g. inside the tube wall of the measuring tube 11 arranged, measuring electrodes are used. Each of the measuring electrodes 14, 15 seizes an electrical potential e, e

14 15, which is induced in operation due to the Faraday law in the fluid flowing therethrough. As shown in FIG. 1 b, during operation, the measuring electrodes 14, 15 are connected at least temporarily to an inverting or non-inverting input of a differential amplifier 22. Thus, a potential difference of the two potentials e, e tapped off from the measuring electrodes 14, 15, serving as measuring signal u, is formed

14 15 corresponds with a built-up in the fluid flowing through the voltage and thus also with the physical quantity to be measured. The applied to the measuring electrodes 14, 15 potentials e, e are usually approximately in the range of 10 mV to 100 mV. It should be mentioned at this point, however, that the potentials but also on separate measuring channels tapped, esp. Also individually digitized, can, see. See also US-A 59 07 103. The output signal shown in the embodiment shown differential amplifier 22 u is, as shown in Fig.la, Ib shown schematically, provided in the flow meter evaluation circuit 3.

[059] FIGS. 3 and 4 relate to exemplary embodiments of a switching mechanism designed as an H-bridge circuit. In a first bridge branch is the controlled current path of a first transistor 13, in a second bridge branch of the controlled current path of a second transistor 14 in a third bridge branch of the controlled current path of a third transistor 15 and in a fourth bridge branch of the controlled current path of a fourth transistor 16 This structure results in four corner points 2a, 2b, 2c, 2d of the H circuit: The transistors 13, 14 through the corner 2c, the transistors 14, 16 through the corner 2b, the transistors 15, 16 through the corner 2d and Transistors 13, 15 connected by the corner 2a with each other. A first bridge diagonal is located between the corner points 2a, 2b and a second bridge diagonal between the corner points 2c, 2d. In the second bridge diagonal coil assembly 1 is at least one field coil of a magnetic field system of the Durchflußaufnehmers set, i. a first and a second terminal of the coil assembly is connected to the corner 2c and 2d, respectively.

[060] In operation, in cooperation with the coil arrangement as a rectangular

Modulator acting circuit arrangement are either the first and the fourth transistor 13, 16 or the second and the third transisor 14, 15 simultaneously conductive controlled. Thus, in the first case (transistors 13, 16 conducting) a (positive voraus¬ set) current from the corner 2a to the corner 2b through the coil assembly 1 and in the direction indicated by the non-dashed arrow direction. If, in contrast, the transistors 14, 15 are conductive, the same current flows in the opposite direction through the coil arrangement 1, as is illustrated by the dashed line.

The coil arrangement 1 has an inductance L and is part of a magnetic field generating magnetic system of a magnetic-inductive Durchflußaufnehmers, which is not shown, since the mechanical structure of such transducer has long been known in the art, see. The aforementioned US-A 42 04 240. For the invention is only of interest that the magnet system comprises a coil core and / or a pole piece.

[062] As the expert has also long been familiar with, the exciter or

Coil current through the mentioned alternating Leitendsteuern the transistors 13, 16 and 14, 15 is generated so that it is positive in the first half period of a period with a constant first current end value and negative in the second half period with a constant second current end value, which is the same as the first final current value. Under current end value, that constant value of the coil current, e.g. 85 mA, which flows before switching in the other current direction.

The corner point 2c lies in FIG. 3 via a resistor 10 at a circuit zero point SN. The resistor 10 forms a series circuit with the H-circuit 2 and is flowed through by the coil current.

[064] FIG. 3 also shows a controlled voltage source 7, which has a voltage output 7c and determines a positive voltage that is assumed to be positive above the series connection, that is to say between the corner point 2a and the circuit zero point SN, cf. the plus sign at the output 7c. The controlled voltage source 7 is fed by the network via two terminals 7a, 7b; it is also located at an output 7d at the circuit zero point SN. The voltage at the output 7c is applied to the corner 2a via the anode-cathode path of a diode 9. For the circuit zero point SN, a capacitor 12, which has a capacitance C, leads from the cathode of the diode 9 and the corner point 2 a.

In FIG. 4, the sequence of coil arrangement and resistance in the series circuit is reversed: the corner point 2b of the H circuit 2 now lies at the circuit zero SN; By contrast, the resistor 10 'is now arranged between the output 7c of the controlled voltage source 7 and the anode of the diode 9. This results in the advantage that a possible short circuit between the vertices 2c, 2d, that is, for example, in the coil assembly 2, the straight-conducting transistors 13, 16 and 14, 15 can not destroy, since the coil current through the Resistor 10 'is limited.

[066] FIGS. 5 and 6 relate to exemplary embodiments with a respective T-circuit 3 or 3 '. A resistor 22 or 22 'forms a series circuit 4 or 4' with the coil arrangement 1. This is formed in the circuit of Fig. 3 so that the Spulen¬ arrangement 1 via the resistor 22 at the circuit zero point SN, and this is traversed by the coil current. In contrast, in Fig. 4 at the circuit zero point SN, the first terminal of the series circuit 4 '.

[067] A first terminal of the controlled current path of a first switch transistor 25 is connected to a second terminal of the series circuit 4, and 4 ', respectively. A second terminal of this current path is connected to a first output 30c of a controlled voltage source 30 which determines a positive voltage across the series circuit, cf. the plus sign at the output 30c. A first terminal of the controlled current path of a second switch transistor 26 is connected to the second terminal of the series circuit 4 or 4 '. A second terminal of this current path is connected to a second output 30d of the controlled voltage source 30, which determines a negative voltage across the series circuit, cf. the minus sign at the output 30d. The switch transistors 25, 26 are alternately turned on, so that the coil current alternately reverses its direction, as the two arrows on the coil assembly 1 illustrate. Again, the coil current in the first half period of a period is positive with a constant first current end value and in the second half period negative with a constant second current end value, which is the same amount to the first current end value.

In Figs. 5 and 6, the positive voltage at the output 30c of the voltage source

30 is connected across the anode-cathode path of a first diode 31 to the second terminal of the switch transistor 25. From this terminal and the cathode of the diode 31 leads to the circuit zero point SN further comprises a first capacitor 33, which has a capacitance Cl. The negative voltage at the output 30d of the voltage source 30 is applied to the second terminal of the switch transistor 26 via the cathode-anode path of a second diode 32. From this terminal and the cathode of the diode 32 leads to the circuit zero point SN, a second capacitor 34, which has a capacitance C2.

In the embodiments of Figs. 3 and 4 or 5 and 6 forms the mentioned

Inductance L of the coil assembly 1 with the capacitance C of the capacitor 12 and with the capacitors Cl and C2 a respective resonant circuit. This causes the voltage across the series circuit to be resonant-boosted and the coil current to have a steeper rising edge during a rise at the beginning of each half-cycle than if the resonant circuit were not present.

[070] For digitization: A supplied measuring signal u preferably digitized and correspondingly store at least sections of inform digital data sets, so that information about the time course of a portion of the measuring signal u in digital form is available to determine the diagnostic value.

[071] According to the invention, the evaluation circuit 3 also serves to generate a diagnosis value for checking the magnetic-inductive flow meter, which represents a current operating state of the flow meter, but in particular a current operating state of the magnetic field system. For example, the diagnostic values can be used to generate a corresponding alarm when, due to deviations of the diagnostic value from a corresponding, pre-determined reference or threshold value, a defective magnet system has been detected. Such defects may be e.g. vibration-induced breakages of electrical lines and or loosening of the assembled elements of the magnetic field system. In addition, significant, esp. Shock-like, fluctuations in the fluid temperature can lead to thermally induced disturbances of the magnetic field system or at least changes in the calibration of the magnetic field system.

For this, in the illustrated embodiment, the voltage source, as already described in US-A 60 31 740, initially controlled so that in each half-period during a first sub-period of the coil current, which is hereinafter referred to as rise time t, a Voltage initial value, U, generated, the, eg rev drv several times, is higher than a constant regulated voltage end value U during a cont second second period, which is referred to below as the remaining time tc, see. Fig. 5a and 5b. Furthermore, the voltage drop across the resistor 10, 10 'or 22, 22' is attracted to it so that an effect of eddy currents, which are induced in the coil cores and / or the pole shoes during the rise of the coil current and the rising edge of the Delay magnetic field with respect to the rising edge of the coil current, is compensated. This is achieved in that in each half cycle the rise time t of the coil current and the height of the positive and negative rev

Voltage end value U are influenced or regulated in such a way that, on the one hand, after reaching a current maximum Im, no further increase of the coil current occurs, so that the magnetic field already reaches a constant final magnetic field value B corresponding to the constant current end value of the coil current, if the m

Coil current reaches the maximum current Im, cf. Fig. 5a, and that on the other hand, the height of the positive and negative voltage end value U always the constant cont

Current end value Ic of eg 85 mA conditionally. This is achieved in that from the course of the voltage drop at the time of the half-cycle after the current maximum Im of the coil current until reaching the final current value Resistance is formed by at least three successive sampling a correction quantity for the voltage across the H-circuit or the T-circuit in the next half-period.

[073] In Figs. 3 to 6, the transistors 13, 14, 15, 16 of the H circuit and the

Switching transistors 25, 26 of the T-circuit always drawn with non-connected control electrode. This is merely intended to indicate that the control of these transistors in the prior art, for example in the aforementioned US-A 44 10 926, is well described and thus the expert knows what he has to control how. Further, the controlled current path of the transistors 13, 14, 15, 16 and 25, 26 with a freewheeling diode 17, 18, 19, 20 and 27, 28 bridged. Although the transistors mentioned are drawn as bipolar transistors, it is of course also possible to use field-effect transistors, in particular insulated-layer field-effect transistors.

In FIG. 5a, in the ordinate direction over the time t, the abscissa is the course of the

Coil current I and the induction B of the magnetic field applied for a whole period. In the first half-cycle with positive coil current I, it is shown that the half-period duration consists of a rise time ta and a residual duration tc; this is equal to the duration during which the induction B of the magnetic field is constant. During the rise time t rev, on the one hand, the coil current increases

I M steeply, then falls off after reaching a current maximum I m again and reaches later than the induction B its constant current end value, I, see. the only all-gradual decrease of the coil current after the current maximum I. This behavior m of the coil current is due to the initially described, induced in the coil cores and / or the pole shoes eddy currents. The time of reaching the current maximum Im determines the boundary between the rise time t and rev of the remaining time t. On the other hand, during the rise time t, the induction B cont rev first increases almost with identical steepness as the coil current, but the rise later becomes flatter and reaches the end of the rise time ta as the final induction value B m.

By the falling portion of the remaining time t oberhald the curve of cont

Coil current indicated arrows is illustrated that the falling down to the current end value I m coil current waveform at least three times, for example every millisecond, is sampled; In the example of FIG. 5a, the falling part of the coil current is sampled four times. From the samples, the control signal for the controlled voltage source is generated according to the invention, as will be explained in detail yet. FIG. 5b shows in the ordinate direction over time t as abscissa the profile of the voltage acting on the series circuit with the H circuit or with the T circuit. The first peak Us is on resonance peak in the coil arrangement due. The following constant value is the mentioned voltage initial value U, which contributes to the control of the increase of the coil current. Without drv

Voltage overshoot would be only the rev during the entire rise time t

Voltage initial value may be present. During the remaining time t, the cont constant voltage end value U occurs. From Fig. 5b it can be seen that, as already mentioned cont, the voltage initial value U is greater than the voltage end value U drv. cont

3 to 6 is controlled by a controller 41 or 42 or 43 or 44 which is controlled between the resistor 10 or 10 'or 22 or 22' and a control input 7e or 30e Voltage source 7 and 30 is arranged, the control illustrates. Further, the corresponding controller controls the transistors 13, 14, 15, 16 of the H circuit and the switch transistors 25, 26 of the T circuit. The controller 41 or 42 or 43 or 44 is at least partially realized by the already mentioned, appropriately programmed microcomputer or microprocessor. This is preceded by an analog / digital converter which digitizes the voltage drop across the resistor 10 or 10 'or 22 or 22'. Of course, microprocessor and analogue / digital converters are clocked by a clock oscillator.

In the method according to the invention, therefore, first, similar to that in the

US-B 67 63 729 or the flowmeter described in US-A 60 31 740 also the excitation current I at least temporarily driving voltage U to increase the current I, of the voltage driven and flowing in the at least one field coil excitation current IM to the maximum current value, I m, at a first instant, t, of the instantaneous first non-zero instantaneous voltage level, U, raised to the, in particular constant, or constant-regulated, second voltage level, U i. drv

As already mentioned, the coil current I cont flowing during the time period t t is sampled at least in sections, digitized and correspondingly stored in the microcomputer mentioned above. The measured and digitally stored current values can now be readily used for the calculation of the voltage profile according to which the voltage driving the excitation current I is to be adjusted in the future for the next half-cycle of the magnetic field B. Thus, practically an iterative or recursive approximated method is used over several measuring cycles. The overvoltage U drv applied to the coil rev during the period of time t is converted from half period to half period or

Measuring cycle to measuring cycle successively tuned so that the coil current IM steadily drops during the remaining period t of each half-period against a cont constant current end value I out. In this respect, attention is drawn to the fact that the time period t rev during which the overvoltage U drv at the coil is applied, is dimensioned so that the measured coil current IM no longer increases during the period t. If there is an increase, this is a cont

Signs that the time span t was too short. As a result, the measuring and operating circuit 2 will rev the time t during the subsequent rev

Half periods as long as appropriate amounts increase until the measured excitation current I has the desired behavior.

For example, the microcomputer may correspond to the one shown in FIG

Programming soot diagram are shown in the appropriate function and decision blocks. Lower case letters provide digital signals that are important to the soot diagram. By means of the aforementioned analog-to-digital converter, the voltage drop at the resistor is digitized, so that a coil current I representing digital signal i is formed. This is supplied to the input of a maximum detector 61 and the input of a gate 62, which is also fed with a maximum signal coming from the maximum detector 61 in the. The gate stage 62 forwards only those portions of the digital current i representing the coil current as current samples s, which are later than the occurrence of the maximum signal.

[081] A first decision stage 63 checks consecutive, adjacent ones

Current samples s under the criterion of whether the later sample is greater than the former, so if the coil current has increased between two samples, and if applicable, at a YES output Y is a control signal y and, if appropriate, at a NO output N a control signal n from. The control signal y causes a pulse duration stage 64 to extend the rise time ta, and the control signal n causes the pulse duration stage 64 to extend the remaining time t. An exit of the cont

Pulse duration stage leads to the voltage source 7 or 30.

[082] A second decision stage 65 continuously checks the current samples s below that

Criterion, whether a current sample s is greater than, equal to or less than a current reference value ir, which is proportional to the current end value I and determines him. The decision stage 65 outputs, if appropriate, a control signal g at a GROS SER output G, a control signal gl at an EQUAL output GL or a control signal k at a SMALLER output K. These control signals g, gl, k are the voltage source 7 and 30, optionally with the insertion of a digital / analog converter, supplied and act on the output from her in a period of Spu¬ lenstroms voltage initial value Ua such that it Increased control signal g in the following period or that leaves it the control signal gl in the following period unchanged or that it reduces the control signal k in the following period. [083] During the actual measurement of the volumetric flow, both the voltage be constant over the coil arrangement as well as the current flowing through the coil arrangement Er¬ current I, since the stability of the measuring signal is also adversely affected inter alia by the inductive coupling of the coil assembly to the measuring electrodes.

[084] Ideally, in both half-periods the current end value of the true coil current I is essentially constant, opposite and equal in magnitude. In practice, however, this is not the case with the flow meter described in US Pat. No. 6,763,729 or US Pat. No. 5,131,740, after a considerable time has passed after the beginning of the time span t. Thus, the time window for during the cont

Time span T of possible exact measurements but practically limited to almost half of a half period. Due to the eddy currents occurring in the pole shoes and coil cores, this ideal case does not occur in reality. Rather, the excitation current I measurable outside the coil arrangement always corresponds to one

M

Current sum, which consists of the true coil current I and the eddy current I

L eddy is composed. However, since only the excitation current I is used by the measuring and operating circuit 2 as a control variable, although the excitation current I M is constant, but not necessarily the constant magnetic field B to be kept constant for the measurement, which passes through the measuring tube. A corresponding equivalent circuit diagram of the coil arrangement is shown in FIG. [085] In the method according to the invention are advantageously in the

Magnetic field system induced eddy currents further minimized by not only an overvoltage U to the coil assembly for the previously determined reference rev

Time span T = t - t is applied until reaching the predetermined current maximum I rev 2 1 m, wherein the overvoltage is so dimensioned that the excitation current I flowing through the Spulen¬ arrangement after the expiry of the reference time period T steadily

M rev falls against a substantially constant current end value I, but after the previously at least approximately determined sequence of the reference time T for rev a predetermined, esb. Equally iteratively determined time period T = t - 1 a

Short 3 2 the excitation current I and thus also the magnetic field

M voltage U is applied to the coil assembly 1, wherein the time period T

Short Short is dimensioned such that the influence of the eddy currents induced during the switching process in the coil arrangement is approximately compensated but at least considerably reduced. The effect of this method can be seen in FIG. 9, in which the solid line indicates the course of the measuring current as it appears in the solution according to the invention; whereas the dotted line shows the temporal behavior of the exciting current in the solution known from the prior art. In the solution according to the invention, the constant current end value I cont is practically reached when the time T Short has elapsed; in the known Solution is that the constant current end value I first becomes a much later cont

Time reached within the period T. During a first change-over switching operation, the measuring and operating circuit 2, as already explained above, applies a corresponding countervoltage U for a currently predetermined period of time T.

Short G to the coil arrangement. Subsequently, the measuring and operating circuit detects a plurality of current measured values within the predetermined period T. For example, cont both the period of time, T, and the time period T can be successively optimized during operation by means of, in particular rev., Short iterative, approximation methods. In the event that at the end of the currently valid time period T, the current end value I is not reached,

Short c is the period T increased accordingly, for example, one in advance

Short certain suitable increment; in the event that before the expiration of the period T of

Short

Current end value I is already reached, the time period T gff. again some c short shorts. Due to the interplay of applied overvoltage and subsequently briefly applied counter-voltage U, the eddy currents

Short effectively compensated and thus their decay time can be significantly reduced. It should also be mentioned at this point that the relevant time periods T, T, T, as rev short cont already indicated, both on the basis of a sample of the exciter current digital data as well as directly by means of the excitation current IM and / or of this driving voltage accordingly triggered timing circuits can be determined.

[086] A third decision stage implemented in the microcomputer checks - assuming that the voltage U is currently kept constant - continuously the current samples s on the criterion of whether successive current samples s signal a constant excitation current I or not. In the event that the assumption of a constant voltage appears to be uncertain, their actual instantaneous time course, for example likewise informal of digital samples, allow corresponding factors to flow into the decision. If appropriate, the decision stage outputs at a KONSTANT output a control signal which signals a constant exciter current I M, as long as one more

Lowering the excitation current I is detected under a set for the constancy of the excitation current I change limit.

[087] Based on the aforesaid, which during each half period for the

In order to improve the readability and also of the diagnostic value, the following method is additionally proposed for generating the actual measuring time but also the diagnostic value for the maximum time available to the optimizing method.

The voltage U currently driving the exciting current I becomes a second one

Time, t 2, from the second voltage level, U drv, to a, esp. Constant or constant-regulated, third voltage level, U short, changed to a lowering of the Current intensity of the excitation current I currently flowing in the at least one field coil, from the maximum current value I, to an exciter current I

M m given, especially constant, current end value, I reach. In this case, the third voltage level, U, is chosen smaller than the second voltage level, U. To determine

Short rev of a third time, t, when the final current value, I, is reached.

3 c spondiert the excitation current I we detected at least temporarily. Based thereon, a first time constant, T, for the magnetic field system, which with a

Short, between the second time, t, and the third time, t, lying time period, t - 1, corresponds, and / or a second time constant, T + T, for

3 2 rev Short determines the magnetic field system corresponding to a time period between the first time, t, and the third time, t, t -t. For determining a diagnostic value finally representing a current operating state of the flow meter, the determined first time constant, T, with

Short a predetermined first reference value, T, and / or the determined second Zeit¬ constant, T rev + T Short, compared with a predetermined second reference value, T 2ref, accordingly.

[089] According to one embodiment of the invention, provision is made for the measuring and operating circuit to short-circuit the coil arrangement during the time period T Short.

Alternatively, it is proposed that the measuring and operating circuit reverses the direction of the current flowing through the coil arrangement during the time period t.

[090] According to a further embodiment of the invention, it is provided that the measuring and operating circuit measures the time span t in such a way that the signal passing through the coil

Short arrangement flowing excitation current at the end of the period t at least approximately short-term, the constant current end value, I, has reached.

Claims

claims

1. A method for testing a magnetic-inductive flow meter with a measuring tube for guiding a fluid to be measured and with a magnetic field system, the at least one at least temporarily by an excitation current Ergerstrom (I) through-flowing field coil for generating a fluid at least The method comprises the following steps: changing a voltage (U) which drives the excitation current (I) at least temporarily, at a first instant, t, from a current, in particular non-zero, first one Voltage level, U, on one, esp. Constant or constant-regulated, second cont

Voltage level, U, to increase a current, I, that of the voltage drv

(U) driven and flowing in the at least one field coil Erre¬ gerstroms (IM) to a maximum current value, I m, wherein the second voltage level, U, is greater than the first voltage level, U, -change drv cont the den Excitation current (I) currently driving voltage (U) at a second time, t, from the second voltage level, U, to a, esp.

2 drv constant or constant-regulated, third voltage level, U, for lowering

Short of the current intensity of the currently flowing in the at least one field coil Er¬ current (I) from the maximum current value, I, to one for the excitation current

M m

(I) predetermined, esp. Constant, current end value, I, wherein the third Spannungs¬ nungshöhe, U, is smaller than the second voltage level, U, -

Short rev

Detecting the excitation current (I) at least temporarily for determining a third time, t 3, which corresponds to reaching the final current value, I c, -

Determining a first time constant, T, for the magnetic field system with

Short of a time period between the second time, t, and the third time, t, t -t corresponds, and / or determining a second time constant, T + T, for the magnetic field system, with a between rev Short the first time, t, and the third time, t, lying time period, t -t, corresponds, and - comparing the determined first time constant, T,

Short with a predetermined first reference value, T, and / or comparing the Er¬ averaged second time constant, T + T, with a predetermined second rev Short

Reference value, T, for determining a current operating state of the

2ref

Flow meter representing diagnostic value.

2. Method according to the preceding claim, wherein the third voltage level, U

, smaller than the first voltage level, U.

Short cont

3. The method according to any one of the preceding claims, wherein the exciting current

(I) at least temporarily driving voltage (U) is changed such that whose second voltage level, U, during one between the first drv

Time, t, and the second time, t, lying period, t -t, at least temporarily, esp. Also immediately before the second time, is substantially constant. 4. The method according to any one of the preceding claims, wherein at least one

Field coil of the coil arrangement for setting the voltage (U) to the third

H

Voltage level, U, at least temporarily shorted.

Short